The track density of magnetic disks has been increased in recent years in order to raise the recording density of hard disk drives. Higher recording density is required in disk drive spindle motors with such high track density. In particular, if non-repeatable runout (referred to hereinbelow as “NRRO”) is large, off-track events during tracking will be created. Therefore, NRRO should be minimized. In view of this, spindle motors with hydrodynamic bearing devices were developed as motors in which high-speed rotation and reduced noise are ensured in addition to high rotational precision.
FIG. 5 depicts a conventional hard disk drive with a hydrodynamic bearing device.
The base end portion of a shaft 2 is fixedly press-fitted into a base 1 as the fixed side, and a thrust flange 3 is provided to the leading end portion of the shaft 2 to form a shaft unit 18, as shown in FIG. 5(a). A sleeve 5 is provided to the external peripheral portion of the shaft 2, and a thrust plate 4 is provided to the sleeve 5 in an opposing arrangement with the thrust flange 3 to form a sleeve unit 17.
Pressure generating grooves 15a and 15b are formed in the internal peripheral portion of the sleeve 5, and a pair of radial bearing portions 14a and 14b is formed as shown in FIG. 5(b). Pressure generating grooves (not shown) are also formed in both surfaces of the thrust flange 3, yielding a thrust bearing (not shown). The gap between the journal 18 and the sleeve unit 17 is filled with incompressible oil 16 to allow the sleeve unit 17 to be rotatably supported.
A coil 10 wound on a stator core 9 is mounted on a wall 1a provided to the external peripheral portion of the shaft 2. A hub 6 for mounting magnetic disks 13 is fixedly press-fitted around the outside of the sleeve 5, and a magnet 7 is mounted via a yoke 8 on the inside of the external peripheral portion of the hub 6 so as to face the stator core 9. A plurality of magnetic disks 13, which are mounted via a spacer 12 around the outside of the hub 6, is attached by the fastening force of a screw 19 via a clamp 11.
When the coil 10 is energized, an electric-current magnetic field is generated between the coil 10 and the magnet 7, the sleeve unit 17 is rotatably driven, lifting force is generated in the sleeve unit 17 by the pumping action of the radial bearing portions 14a and 14b and the pressure generating grooves formed in the thrust bearing portion, and the sleeve unit 17 is rotated without contact via the journal 18 and oil 16.
Runouts are induced in the magnetic disk device thus configured because the load applied to the radial bearing portions 14a and 14b during rotation varies depending on the formation position of the pair of radial bearing portions 14a and 14b formed around the inside of the sleeve 5, and on the center of gravity position of a rotating body that comprises the sleeve unit 17, the magnetic disks 13 mounted thereon, the spacer 12, the clamp 11, and the like.
In view of this, the assembly is commonly configured such that the length of the radial bearing portions 14a and 14b varies in the axial direction, as disclosed in Japanese Patent Application Laid-open H8-335366. The length of the radial bearing portions 14a and 14b in the axial direction will be referred to hereinbelow as “the bearing length.” Specifically, adjustments are made such that the bearing length L1 of the radial bearing portion 14a formed on the side facing the leading end of the shaft 2 is greater than the bearing length L2 of the radial bearing portion 14b formed on the side facing the leading end portion of the shaft 2, balance is achieved between the bearing rigidity of the radial bearing portions 14a and 14b and the center of gravity position G, and runouts are reduced.
This structure does not present any problems in existing hard disk drives, but even higher rotational precision and lower NRRO will be required in the future as hard disk drives become more compact and are rotated at higher speeds. The following are believed to be the reasons for the occurrence of NRRO.
In the radial bearing portions 14a and 14b provided with different bearing lengths L1 and L2 in the above-described manner, the herringbone-shaped, pressure generating grooves 15a and 15b are designed such that the groove depths, groove angles, and groove width ratios are equal to each other. For this reason, eccentric angles, which are the angles between the forces exerted on the sleeve 5 by dynamic pressure and the position in which the gap between the external peripheral surface of the shaft 2 and the internal peripheral surface of the sleeve 5 during rotation is at its minimum, is such that the eccentric angle of the radial bearing portion 14a becomes greater than the eccentric angle of the radial bearing portion 14b. As a result, rotational precision is reduced and NRRO increased by rotation during which the center line of the shaft 2 and the center line of the sleeve 5 are inclined relative to each other.